This invention relates generally to nuclear reactors and more particularly, to a system for monitoring reactor instability and controlling reactor suppression.
In known types of nuclear reactors, such as boiling water reactors (BWR), the reactor core includes a plurality of fuel bundles arranged in an array capable of self-sustained nuclear fission reaction. The core is contained in a reactor pressure vessel (RPV). The typical core is submerged in a liquid such as water, which serves as both a core coolant and a neutron moderator. Each fuel assembly includes a flow channel through which water is pumped upwardly from a lower plenum to an upper plenum. A plurality of control rods containing neutron absorbing material are insertable between the fuel bundles to control the reactivity of the core. To monitor core conditions, it is common practice to distribute neutron detectors both radially and axially throughout the core. The signals from these neutron detectors are utilized to monitor the power density of the core and to initiate corrective actions, including reactor suppression, in the event of detected instability.
A nuclear reactor operates under three distinctly different stability regimes. These regimes are a stable reactor state, a reactor instability threshold state and an unstable reactor state. Reactor instability occurs when fuel cladding heat flux and channel coolant flow rates deviate from steady state conditions during power oscillations significantly above the normal neutron noise level. Reactor instability must be monitored to prevent damage to the core and to within fuel safety limits and can be accomplished by either detecting and suppressing instability induced power oscillations, or preventing them altogether.
Known "detect and suppress" systems are based on a common approach. Generally, neutron detectors, for example, local power range monitors (LPRMs), are placed within the core of the reactor. The neutron detectors generate signals indicative of reactor thermal-hydraulic oscillation frequencies. These signals are characteristic of power oscillations of the reactor. Some known "detect and suppress" systems monitor successive oscillation periods and provide a final oscillation amplitude trigger to generate a reactor trip signal. Another known system monitors of an oscillation growth rate limit and if the limit is exceeded, the system generates a reactor trip. Yet another known system compares the neutron detector signal to an amplitude trip setpoint and if the setpoint is exceeded, a reactor trip signal is generated.
Although the known systems generally provide satisfactory results, such systems permit the development of significant power oscillations prior to actuation of the suppression function. As a result, rigorous analysis of minimum critical power ratio (MCPR) performance in the presence of core power and flow oscillations is necessary to prevent exceeding the fuel safety limits or damaging the core. In particular, the transient thermal-hydraulic behavior within the limiting fuel bundle must be related to neutron flux oscillations observed by various neutron detectors.
Another shortcoming of known systems is the potential spatial effects related to the oscillation mode and its impact on the magnitude of the oscillation as observed by any given neutron detector. Combinations of neutron detector signals must be related to a reactor trip setpoint for each of the detection methods. As a result, the known systems are time consuming to determine the appropriate trip levels and allow significant power oscillations to occur prior to suppression. Additionally, current "detect and suppress" systems are still evolving since the quantification of MCPR performance as a function of power oscillation scenarios for the full spectrum of core designs and operating conditions is extremely challenging.
It would be desirable to provide a system that, prior to the development of significant power oscillations, facilitates suppression of the nuclear reactor upon reaching the threshold of reactor instability.